Goal:
  
  G1 : t
  G2 : t
  H : invariants G1
  H0 : invariants G2
  l : labelling
  HGl : correct_labelling G1 l
  embed : full_subgraph G1 G2
  acG2 : acyclic_no_loop G2
  Gl := relabel_on G1 G2 l : t
  l' := to_label ∘ lsp Gl (s Gl) : V.t -> nat
  x : VSet.elt
  xin1 : VSet.In x (V G1)
  p : PathOf G1 (s G1) x
  lsp_eq : lsp (relabel_on G1 G2 l) (s G1) x = Some (diff l (s G1) x)
  ls : l (s G1) = 0%nat
  ============================
   (to_label (Some (Z.of_nat (l x))) = l x)


DEBUG:Going to execute:
apply : Nat2Z.inj